Check the appropriate box to
designate the rule pursuant to which this Schedule is filed:

☐ Rule 13d-1(b)

☐ Rule 13d-1(c)

☒ Rule 13d-1(d)

*

The remainder of this cover page shall be filled out for a reporting persons initial filing on this form
with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed to be filed for the purpose of Section 18 of the Securities
Exchange Act of 1934 (Act) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

CUSIP No. 05988J103

Schedule 13G

Page 1 of 4

1

Names of Reporting Persons

Henry R. Kaestner

2

Check the Appropriate Box
if a Member of a Group

(a) ☐ (b) ☐

3

SEC Use Only

4

Citizenship or Place of
Organization

United
States

Number of

Shares

Beneficially

Owned by

Each

Reporting

Person

With

5

Sole Voting Power

2,059,747

6

Shared Voting Power

11,862

7

Sole Dispositive Power

2,059,747

8

Shared Dispositive Power

11,862

9

Aggregate Amount Beneficially Owned by Each Reporting Person

2,071,609

10

Check if the Aggregate
Amount in Row (9) Excludes Certain Shares

Not Applicable

11

Percent of Class
Represented by Amount in Row 9

13.8%

12

Type of Reporting
Person

IN

CUSIP No. 05988J103

Schedule 13G

Page 2 of 4

ITEM 1.

(a)

Name of Issuer:

Bandwidth Inc. (the Issuer)

(b)

Address of Issuers Principal Executive Offices:

900 Main Campus Drive, Raleigh, NC 27606.

ITEM 2.

(a)

Name of Person Filing:

This statement is filed on behalf of Henry R. Kaestner (the Reporting Person).

(b)

Address or Principal Business Office:

The business address of the Reporting Persons is c/o Bandwidth Inc. 900 Main Campus Drive, Suite 500, Raleigh, North Carolina
27606.

(c)

Citizenship of each Reporting Person is:

The Reporting Person is a citizen of the United States.

(d)

Title of Class of Securities:

Class A Common Stock, par value $0.001 per share (Class A Common Stock).

(e)

CUSIP Number:

05988J103

ITEM 3.

Not applicable.

CUSIP No. 05988J103

Schedule 13G

Page 3 of 4

ITEM 4.

Ownership.

The ownership information below represents beneficial ownership of Class A Common Stock of the Issuer as of
December 31, 2018, based upon 12,898,762 shares of Class A Common Stock outstanding as of December 14, 2018 and assumes the conversion of the Class B Common Stock, par value $0.001 per share (Class B Common
Stock) of the Issuer held by the Reporting Person into shares of Class A Common Stock of the Issuer on a one-to-one basis.

(a)

Amount beneficially owned:

The Reporting Person is the record holder of (i) 2,845 shares of Class A Common Stock and (ii) 2,056,902 shares of
Class B Common Stock. The Reporting Person also may be deemed to share beneficial ownership of 11,862 shares of Class B Common Stock he holds as custodian for his children.

(b)

Percent of Class: 13.8%

(c)

Number of shares as to which such person has:

(i)

sole power to vote or to direct the vote: 2,059,747

(ii)

shared power to vote or to direct the vote: 11,862

(iii)

sole power to dispose or to direct the disposition of: 2,059,747

(iv)

shared power to dispose or to direct the disposition of: 11,862

ITEM 5.

Ownership of Five Percent or Less of a Class.

Not applicable.

ITEM 6.

Ownership of More than Five Percent on Behalf of Another Person.

Not applicable.

ITEM 7.

Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on By the
Parent Holding Company.

Not applicable.

ITEM 8.

Identification and Classification of Members of the Group.

Not applicable.

ITEM 9.

Notice of Dissolution of Group.

Not applicable.

ITEM 10.

Certification.

Not applicable.

CUSIP No. 05988J103

Schedule 13G

Page 4 of 4

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true,
complete and correct.